Polyesters of benzene dicarboxylates and glycols are widely used in thermoplastic fibers, films and molding applications because of their excellent physical properties and surface appearance. Polyethylene terephthalate (PET), polypropylene terephthalate (PPT) and polybutylene terephthalate (PBT) are the more common commercial polyesters within this class of polymers, generally referred to as "polyalkylene terephthalates".
Polyalkylene terephthalates commonly are prepared by one of two routes: (1) transesterification of a dialkyl terephthalate diester (e.g., dimethyl terephthalate DMT) with a diol (e.g., ethylene glycol) to form an intermediate (e.g., bis-hydroxyethyl terephthalate BHT), followed by polycondensation to form the polyalkylene terephthalate (e.g., PET); or (2) by direct esterification of terephthalic acid (TPA) with a glycol (e.g., ethylene glycol), to produce the same BHT intermediate followed by polycondensation to form the polyalkylene terephthalate (e.g., PET). The term "(trans)esterification" is used in this document to refer either to esterification, transesterification, or both.
Cationic dyeable polyesters (CDPET) are produced by using small amounts of sulfonated isophthalate metal salts or their bis-glycolate esters as co-monomers along with DMT or TPA. Fiber made from CDPET copolymer gives brilliant shades on dyeing with basic/cationic dyes and also dyes with disperse dyes to deeper shades and at lower temperatures than fibers made from PET. Typically these cationic dyeable polyesters are made by incorporating up to about 3% of 5-sulfo isophthalic acid (SIPA) or its dimethyl or bis-glycolate ester, generally as the sodium salt. The use of the bis-glycolate ester of 5-sulfo isophthalate sodium salt is particularly preferred because of greater manufacturing efficiency, i.e., shorter polycondensation time for making the sulfo-modified polyalkylene terephthalate.
Catalysts are used to speed the above (trans)esterification and polycondensation reactions. The same or different catalyst may be used for both steps. Many current commercial processes use manganese or zinc salts as the catalyst for the (trans)esterification step. Antimony, in the form of a glycol solution of antimony oxide, is used as the polycondensation catalyst. Organic titanates such as tetraisopropyl titanate and tetrabutyl titanate are also known to be effective (trans)esterification and polycondensation catalysts for preparing polyalkylene terephthalates.
For incorporating sulfo-modified monomers in the above compositions, a bis-glycolate ester of 5-sulfo isophthalate sodium salt can be used. It can be prepared by esterification of 5-sulfo isophthalate sodium salt (NaSIPA), or by transesterification of the dialkyl ester of 5-sulfo-isophthalate sodium salt such as, for example, dimethyl ester of 5-sulfo-isophthalate sodium salt (NaDMSIP). Either tetraisopropyl titanate (see JP 2-117,959) or manganese acetate (see JP 49-117,446 and U.S. Pat. No. 3,899,470) are typically used as (trans)esterification catalyst to enable complete conversion to the bis-glycolate ester. However, these catalysts have a tendency to impart an undesirable yellow tinge to the resulting cationic dyeable polyalkylene terephthalate. There is a need for an improved catalyst for the process of manufacturing the above bis-glycolate ester by (trans)esterification of 5-sulfo isophthalic acid or ester metal salts.